The Importance of Learning
Milo tells himself that learning things is a waste of time, but his view changes after his adventures. He realizes that learning is valuable because it makes life more exciting, valuable, and comprehensible. It keeps relationships harmonious, people safe, the economy humming along, and more. Learning may be difficult sometimes, but it is worth it because you truly get to know yourself and your world. Learning is a world we enter, and it is worth traveling there as often as possible.
Friendship
Milo is the protagonist of the story, but he would not have been able to achieve the aims of his quest if he did not have steadfast friends like Tock and the Humbug at his side. Indeed, Milo isn't even the smartest or strongest character in the trio; Tock is wiser and stronger and actually saves their lives multiple times. Milo's pre-tollbooth life isn't known except for his boredom, but it is fair to assume that he did not have many friends. In this place, though, he learns that friends are valuable for their physical support as well as their emotional support. Tock helps Milo with difficult concepts and is loving and loyal no matter what happens.
The Dangers of Specialization
King Azaz thinks words are the most important things in the world. The Mathemagician counters that numbers are. The Soundkeeper decides that only beautiful sounds can be heard and banishes all sounds when she cannot get her way. These narrow, specialized focuses are dangerous and limiting. They lead to corruption and a desire for power to assert one's will regardless of what is good for everyone else. Words and numbers in particular should coexist; without that unity, life would be incomplete. People need both in order to fully comprehend, engage in, and appreciate their world and themselves.
Perseverance
Although King Azaz and the Mathemagician don't tell Milo at the beginning that his journey is impossible, Milo must to some extent feel that. He and his friends keep getting bogged down as they continue along their way to rescue Rhyme and Reason. They end up on the Island of Conclusions, waste hours on the Terrible Trivium's tasks, try and fail to reach Infinity, find themselves in the clutches of demons, answer dozens of mundane questions about themselves, and more. Without a willingness to persevere, to complete the journey, and to work through the difficult times, Milo, Tock, and the Humbug would not have succeeded.
Openmindedness
Milo begins the novel with an extremely close-minded attitude. He has decided that school and learning are useless, and seems unlikely to alter that view anytime soon. However, by the end of the novel he has gained a new, more open-minded approach to learning; he recognizes its usefulness, merits, and inherent excitement. He also learns to be open-minded about people (or sometimes in this novel, creatures). Tock initially scares him, as does the Spelling Bee. He is unsettled by Faintly Macabre, but she turns out to be a kindly old woman who helps them with their quest. Indeed, the reader also learns to be open-minded, for these characters are not always what they seem. The Humbug is a blustering, pompous pest at first, but turns out to be a wonderful ally. Many of the characters that show up to fight the demons at the end were not necessarily that likeable when they first appeared. Overall, Juster implies that most people have virtue and value, and that most things the world has to offer do as well.
The Importance of Childhood
When Milo meets Alec, he is fascinated how the young boy grows downward instead of up. He tries it himself and then concludes, after tumbling, that he wants to think as a child for now. This is an important moment because Juster is indeed suggesting that the curious, open-minded, enthusiastic, and tolerant mind of a child is an excellent thing to possess. Juster never talks down to his reader and he does not make Milo immature or stupid; the "mind of a child" is not intended to indicate naivete or babyishness. Rather, children are able to absorb, to pursue, and to embrace things in a more open and prolific way than adults can.
Avoiding the Doldrums
Early on in his journey, Milo gets stuck in the Doldrums, a place where doing practically nothing is de rigueur, and where laughing and thinking are anathema. This place is familiar to most people and Juster himself has indicated that he spent his childhood in a place like this. The Doldrums enervate, deaden, and destroy. They are "easy" to inhabit but the things that make life meaningful vanish. Milo gets out of the Doldrums with help from a new friend and with thinking, and he does not return. Children (and adults) reading this may recognize the Doldrums and Juster implies that finding the beauty in words and numbers and sounds and the wider world can help keep them away from there.
